In the prior art, fabrication of a graded-refractive-index optical fiber is well known. For example, U.S. Pat. No. 5,760,139 and U.S. Pat. No. 5,783,636 disclose seven methods for fabricating a graded-refractive-index optical fiber. In such an optical fiber, a dopant is so distributed in a fluoropolymer as to have a concentration gradient in the direction from the center to the periphery. Preferably, it is an optical fiber wherein the dopant is a material having a higher refractive index than the fluoropolymer, and the dopant is so distributed as to have a concentration gradient such that the concentration of the dopant decreases in the direction from the center of the optical fiber to the periphery. Hence, a graded refractive index optical fiber is produced by arranging the dopant at the center and diffusing the dopant toward the periphery. In other cases, a graded refractive index optical fiber is formed wherein the dopant is a material having a lower refractive index than the fluoropolymer, and the dopant is so distributed as to have a concentration gradient that the concentration of the dopant decreases in the direction from the periphery of the optical fibers to the center. Hence, a graded refractive index optical fiber is produced by diffusing the dopant from the periphery toward the center.
The above-referenced patents disclose the following seven methods for fabricating graded index plastic optical fiber. A first method comprises melting the fluoropolymer, injecting the dopant or a fluoropolymer containing the dopant at the center of the melt of the fluoropolymer, and then molding the melt while or after diffusing the dopant. In this case, the dopant may be injected at the center not only so as to form only one layer but also so as to form multiple layers. The molding is carried out by melt-extrusion, which is suitable for forming a rod-like body material such as a preform of an optical fiber, or by melt-spinning, which is suitable for forming an optical fiber.
A second method comprises dip-coating the dopant or the fluoropolymer containing the dopant on a core formed from the fluoropolymer by melt spinning or drawing.
A third method comprises forming a hollow tube of the fluoropolymer by using a rotating glass tube or the like, filling in the polymer tube with a monomer phase which gives the dopant or the fluoropolymer which contains the dopant, and then polymerizing the monomer phase while rotating the polymer tube at a low speed. In the case of interfacial gel polymerization, at the polymerization step, the tube of the fluoropolymer swells up in the monomer phase and forms a gel phase, and the monomer molecules are polymerized while preferentially diffusing in the gel phase.
A fourth method wherein two kinds of monomers with different reactivities are used, one of which is a monomer which forms the fluoropolymer, and the other is a monomer which forms the dopant, are used, and the polymerization reaction is carried out so that the compositional proportion of the resulting fluoropolymer to the resulting dopant varies continuously in the direction from the periphery to the center.
A fifth method comprises hot-drawing or melt-extruding a mixture of the fluoropolymer and the dopant obtained by homogeneously mixing them or by homogeneously mixing them in a solvent and then removing the solvent by means of evaporation, into fibers, and then (or immediately after the formation of the fibers) bringing the fibers into contact with an inert gas under heating to evaporate the dopant from the surface and thereby forming a graded refractive index. Alternatively, the fibers are immersed in a solvent which does not dissolve the fluoropolymer but dissolves the dopant so as to dissolve out the dopant from the surface of the fibers so that a graded refractive index is formed.
A sixth method comprises coating a rod or a fiber of the fluoropolymer with only the dopant which has a smaller refractive index than the fluoropolymer or with a mixture of the fluoropolymer and the dopant, and then diffusing the dopant by heating to form a graded refractive index.
A seventh method comprises mixing a high-refractive-index polymer and a low-refractive-index polymer by hot-melting or in a state of a solution containing a solvent, and diffusing them in each other while (or after) multilayer-excluding in a state that each has a different mixing ratio, to eventually obtain a fiber having a graded refractive index. In this case, the high-refractive-index polymer may be the fluoropolymer, and the low-refractive-index polymer may be the dopant.
In all of the above fabrication methods, optical transmission bandwidth is determined by a post-manufacturing test step after the cladding and buffer have been added to the fabricated core of the optical fiber and all diffusion processes are completed. If the transmission bandwidth does not meet the required specifications, the optical fiber must be discarded. This represents a significant problem in the prior art method of fabricating plastic optical fiber.